The field of the present invention relates generally to gasification systems and, more particularly, to systems and methods for removing moisture from solid feedstock within a gasification system.
The gasification of solid feedstocks and the subsequent combustion of hydrocarbon components from the feedstock in a gas turbine engine are known. When coal is used as the feedstock, some known gasification processes require a relatively dry (i.e., low moisture content) coal be used because of known difficulties associated with conveying moist solids, and the inherent efficiency losses associated with moisture that may be present in the coal feedstock. Since almost all commercially available coals contain a certain amount of water, a need exists to dry the coal in an efficient manner prior to gasification. This need becomes even more apparent when using sub-bituminous, lignite, or brown coal feedstocks that often contain between about 20% to about 65% by weight water.
Some known methods for drying solid feedstocks in gasifiers involve sweeping very hot gas through a solids grinding mill. In such systems, in order to efficiently evaporate the excess moisture, the drying gas must be maintained at a temperature well above the boiling temperature of water at the system operating pressure, normally between 300° F. and 900°. Various systems have been used in the past to create a drying gas medium that can be used to remove excess water in solids coal feedstocks. However, generally known sources of heating and drying solid feedstock have drawbacks that invariably reduce overall plant efficiency. For example, many known systems may use superheated steam and/or gas turbine extraction air channeled to the heat exchangers for drying, or may use a fuel, such as natural gas or propane, within a direct fired or an indirect fired heat exchangers. In such known direct fired configurations, hot combustion gases are generated using mixtures of air and fuel. Direct firing of those fuels creates pollutant emissions and thus is not an environmentally acceptable method to economically dry a solid feedstock. Other known methods use process steam and/or heated gases supplied from a separate power plant in an indirect fired heat exchanger. Again, the need for separate power plant facilities for providing the necessary heat often is not an economical alternative.
Another known method of drying solids involves burning a portion of the clean synthesis gas (also known as “syngas”) produced through gasification and pass combustion gases over the milled coal as it is transported into a powder bunker or hopper. Milling and drying plants may reduce the overall efficiency of the power generating plant because such plants consume part of the gaseous fuel during the drying process. Another known method obtains drying energy by burning a portion of the milled coal, thereby heating the feed circulating in the drying plant. However, in each of these known methods, the net efficiency of the power generating plant decreases. Moreover, emissions, such as sulfur, from the power plant increase when making drying energy available in such a manner. While various conventional methods exist for drying coal feedstocks, a significant need exists to reduce the inherent thermal inefficiencies in such known processes.